1. Field of the Invention
This invention relates to optical projection systems in general and in particular to color video projector system using transmissive light valves and, non-absorptive polarization control technology. The new video projector system provides improved overall efficiency and higher light output in a compact, easily manufacturable configuration.
2. Brief Description of the Prior Art
Video projection systems have been utilized for some time in a broad range of applications. For example, such systems can be found in the home for consumer use, in command/control centers for military use, as part of CAD/CAM systems, in conference rooms for general business use, in restaurants/bars for group viewing of sporting events, and in commercial aircraft. These applications are reflected already in a number of products in the marketplace at a broad range of price performance points, and the range of different applications will continue to expand. However, light efficiency, light output, size, and cost of such video projector apparatus continue to be problematic factors that are limiting more wide-spread use and customer acceptance.
Several different technologies have been used in the development and manufacture of video projection systems. Principle among them are cathode ray tube (CRT) based systems and the light valve based systems The CRT based systems generate their light outputs by the excitation of phosphors via electron beams. Light valve based systems, on the other hand, modulate beams of light, both spatially and temporally, via light valves. Designers of systems employing both technologies are continually attempting to improve their products. Performance issues of particular importance include light output or intensity (brightness) capability, resolution, overall light efficiency, and size. Ease of manufacture and cost are also important considerations.
CRT based video projection systems project images produced by electron beam excitation of a phosphor, similar to a conventional television picture tube. High video image resolution is easily obtained with such CRT based systems, but achieving high light output, or high brightness in large images is difficult and expensive. Although improvements are being made in excitation efficiencies of phosphor materials for this purpose, there are still practical limits on the amount of light that can be generated by electron excitation of phosphors. As a result, efforts to achieve more light output in CRT based systems have generally resorted to the use of larger CRT's. Consequently, CRT based systems have generally been large and difficult to align and maintain, in addition to being increasingly expensive when high light output is required.
Light valve based systems fall into multiple categories and have utilized oil films, magneto-optical materials, and liquid crystals, as light valves for modulating light beams to write video images into the beams. Commercial color projection systems that use electrically alterable liquid crystals (LC's) as light valves have been a relatively recent occurrence and are of principle interest here. They offer the promise of small size, low cost, ease of alignment, and low maintenance. However, current commercial liquid crystal (LC) systems, particularly those in which light is transmitted through the light valve, are very limited in resolution, light output capability, and overall efficiency, as will be discussed in more detail below.
Current LC light valve based commercial color video projection systems operate on the principle of separating a beam of white light into three polarized red, green, and blue components. Three separate LC light valves positioned respectively in the separate red, green, and blue color light beams are then driven to modulate the color light beam components in such a manner as to impart the desired video images to the color beam components by altering polarizations of pixel portions of the light beams. Separate polarizers are then used in each light path to "analyze" the beam, i.e., to transmit light from pixels of the light valve having the desired polarization characteristics and to block light from those pixels of the light valve that do not have the desired polarization characteristics. The separate beam components are then recombined to form the color video image for projection onto a viewing screen.
Several current commercial LC systems, such as those made by Epson, Sharp, and Toshiba, use light valves that use electrically addressable pixel arrays to modulate the three colored beam components. The image resolution of such LC based systems is limited by the number or density of individually electrically addressable pixels that can be built into an LC light valve. Today, this number is in the 100,000 to 200,000 pixel range, but advances in manufacturing technology can be expected to lead to rapid improvements. These systems have other significant limitations in that they are very inefficient and greatly limited in available light output. These systems typically use about a 200 watt light source and yet only have light output in the range of 100 lumens and an overall efficiency of about 2%. The major advantage of electrically addressable LC systems are their small size and low cost.
There are also LC based systems that use the light output of CRTs to activate the liquid crystal devices. These LCs in turn modulate beams of light in a reflective mode. These systems have been used in high end applications by manufacturers such as the Hughes Aircraft Company. Such systems are capable of very high resolution. However, they are also very complex systems that are further characterized by very high cost, large size, high power dissipation, and other characteristics that make them unsuitable for high volume commercial applications.
In general, the light output and efficiency of LC systems are in a large measure attributed to how the polarized light is created, analyzed, and recombined. Improvements in any of these methods can lead to reduced power and cooling requirements, improved lifetime, smaller size, and lower cost as well as higher light output and efficiency. In short, the competitiveness of these LC based systems is greatly influenced by the design of the polarization control system.
In the Epson, Sharp, and Toshiba systems mentioned above, which are electrically driven LC systems, major light losses are attributable to the methods used to create, analyze, and recombine the polarized light. For example, these systems all employ absorptive sheet polarizer technology for polarizing the light before it enters the LC arrays for modulation and for "analyzing" it after modulation by the LC arrays. Such absorptive polarizing sheets are stained, oriented polymers that pass only light polarized in one plane and absorb the rest. This limits light output and causes a loss in efficiency, since they can only absorb a limited amount of energy before they heat to the point of damage or destruction.
A fairly recent color projection system developed by NEC, as described in a paper entitled, "High Brightness Crystal Light Valve Projector Using a New Polarization Converter," presented at the 1990 SPIE/SPSE Symposium on Imaging and Science Technology, Feb. 11-16, 1990, in Santa Clara, Calif., addresses the efficiency problem by retaining both the "S" and "P" polarizations, converting the P polarization to S, and combining them into one S beam. However, this NEC system still uses sheet polarizers for analyzers and color recombination by wavelength selective thin film coatings to obtain a composite, full color beam. Therefore, it still has the inefficiencies associated with both of these techniques as well as an upper light output limit imposed by the sheet polarizers. Of course, the light output of systems such as this one can be increased by using larger sheet polarizers that can absorb more energy. However, such larger components would increase the overall size and cost of the projector, and would prohibit miniaturization, which is an important developmental goal.
The Hong et al. patent, U.S. Pat. No. 4,191,456, which is an example of one of the more "high-end" video projection systems in terms of cost and complexity, is assigned to Hughes Aircraft Co. and discloses an LC light valve system that uses cathode ray tubes (CRT's) to light-actuate pixels of reflective LC arrays to write video images into blue, green, and red beams. Hong, et al., attempt to solve the sheet polarizer absorption problem by replacing the sheet polarizers with polarizing beamsplitting cubes, which transmit, instead of absorb, the unwanted polarization components. The system also uses color recombination to combine the separate blue, green, and red light beams into a single beam that is passed through the one projection lens, thereby doing away with the need for three individual sets of projection lenses. Hong, et al., uses four (4) polarizing beamsplitters in combination with a number of dichroic mirrors, color trimming filters, and CRT driven reflecting light valves to accomplish the polarization modulation and recombination of the three individual colored light beams. A major deficiency of the Hong, et al., system is that it only makes use of one polarization state of the source light. The other polarization component is unused. The system, therefore, immediately loses 50% of the available light. Coupling and transmission inefficiencies associated with the relatively large number of optical components further reduce the efficiency of the system. The Hong, et al., system also requires the use of three individual CRTs to drive the light valves, thus further increasing the complexity, cost, size, and energy inefficiency of the system.
Jacobson et al., U.S. Pat. No. 4,127,322, represents an improvement over Hong, et al., in that they disclose a polarized video projection system that is able to use both polarization components generated by the light source. Jacobson, et al., also does away with several of the polarizing beamsplitter cubes required in the Hong, et al., system. Disadvantageously, this system, however, still requires at least three individual CRT's, to drive the respective reflective LC light valves, thus, still retaining the size, cost, energy inefficiencies, heat generation, and other problems associated with the use of CRT's.
Finally, one last factor leading to the overall inefficiency of LC type projector systems mentioned above is the inability to perfectly collimate the source beam before passing it through the individual LC arrays. Imperfect collimation is a direct result of the non-point nature of the light source. Such imperfect collimation causes the spot size of the source beam to rapidly diverge, or spread, as the distance from the source increases. Since the energy density of the spot is directly proportional to the square of the spot diameter, the flux through the LC array drops rapidly as the distance from the source increases. Thus, shortening the light path distance from the light source to the LC arrays is critical in achieving maximum efficiency. Similarly, shortening the light path between the LC arrays and the output lens is critical to achieving maximum light coupling to the output lens and thus efficiency for a given lens size.
Therefore, there remains a need for a relatively low cost color video projection system that has high light output and brightness, good contrast, and good resolution, but which is also small, efficient, low power consuming, and relatively easy and inexpensive to manufacture. In fact, it is desirable that products could be derived from such a system design concept that could span the range of applications from small image devices suitable for desk top computer applications to very large light output devices suitable for large screen conference or theater applications. To do so, such systems must be capable of creating and recombining the individual color beam components to produce a composite video image as efficiently and effectively as possible before passing them through a single projection lens. Prior to this invention, there have not been any color video projection systems in a commercially viable configuration suitable for use in cost sensitive markets that combine transmissive or electrically addressable LC based light valves with non-absorptive polarization selective beamsplitter/recombination cubes and other non-absorptive components for the creation, analyzation, and recombination of the light beams. There have also not been any such projection systems that are efficient enough to produce in the range of 1000 lumens or more video light output from a conventional 200 or 300 Watt or similar light source, as opposed to the approximately 100 lumens output available in other transmissive LC-based projection systems.